1. Field of the Invention
The present invention relates to a variable-power eyepiece optical system which is used in combination with an objective optical system of binoculars, telescopes or microscopes.
2. Description of the Related Art
Various types of variable-power eyepiece optical systems having a magnification ratio of about 2 have conventionally been proposed. For example, Japanese Unexamined Patent Publication No. Hei-4-204616 has disclosed an afocal variable-power optical system including an objective optical system, an erecting optical system and a variable-power eyepiece optical system. Further, Japanese Unexamined Patent Publications Nos.Sho-62-134617 and Hei-6-175048 have solely disclosed variable-power eyepiece optical systems, i.e., the variable-power eyepiece optical systems have been disclosed without being accompanied by an objective optical system and an erecting optical system. With respect to each of the above publications, the variable-power eyepiece optical system is formed as a three-lens-group variable-power eyepiece optical system including a negative first moveable lens group, a field diaphragm, a positive second moveable lens group and a positive third immoveable lens group, in this order from the object. According to this arrangement, upon variable-power the first and second moveable lens groups are moved in the opposing directions, and at the same time, the change in diopter is corrected; however, the change in the exit pupil distance (i.e., the eye relief: the distance between the most viewer's eye-side surface and the eye point) cannot be corrected. In particular, if an attempt is made to obtain a higher magnification ratio of more than 2, the change in the exit pupil distance becomes extremely large, so that variable-power causes an eclipse.